Q.78 An actively growing culture of E. coli divides in about 20 min.
Under laboratory conditions, time taken to replicate the entire genome of this bacterium would be about:
(A) 20 min
(B) 40 min
(C) 10 min
(D) 18 min
E. coli divides every 20 minutes, but its genome replication takes ~40 minutes due to overlapping replication rounds. The correct answer is (B) 40 min, as this represents the C-period (chromosome replication time) under lab conditions, distinct from the faster doubling time enabled by multifork replication.
Core Concept
E. coli growing optimally (rich medium, 37°C) has a 20-minute generation time (doubling time), yet replicating its 4.6 Mb circular chromosome requires ~40 minutes (C-period). This is possible because cells initiate new replication rounds before prior ones finish, creating multifork chromosomes with 2–4 replication forks per origin simultaneously.
Option Analysis
| Option | Time | Why Correct/Incorrect |
|---|---|---|
| (A) 20 min | Matches division time | Wrong: This is generation time (τ), not replication time (C). Overlapping allows division despite longer C. |
| (B) 40 min | C-period (replication) | Correct: Lab-measured time for bidirectional forks from oriC to traverse entire chromosome (~1000 kb per fork). |
| (C) 10 min | Half replication | Wrong: Too fast; doesn’t match experimental data. Might confuse with fork speed (~600 bp/sec). |
| (D) 18 min | Arbitrary | Wrong: No biological basis; close to division but ignores multifork reality. |
Detailed Mechanism
Timeline (0–40 min growth cycle):
-
t=0 min: “Grandmother” cell initiates 3rd round at oriC (previous rounds ongoing).
-
t=20 min: Cell divides → each daughter inherits partially replicated chromosomes (1st round ~50% done, 2nd ~started).
-
t=40 min: Daughters complete replication + divide, having initiated their own new rounds at t=20.
Key equation: For τ < C, initiation frequency = 1/τ. At 20 min doubling, cells initiate every 20 min despite 40 min C-period.
E coli genome replication time puzzles test bacterial cell cycle control—why does this bacterium divide every 20 minutes when its entire chromosome takes 40 minutes to replicate? This E coli genome replication time guide reveals the 40-minute C-period answer, perfect for CSIR NET Life Sciences, GATE Biotechnology, and microbiology grad students tackling multifork replication.
The Paradox: 20-Min Division vs 40-Min DNA Copy
Under lab conditions (LB broth, 37°C), E. coli doubles every 20 minutes—faster than its ~40-minute genome replication time (C-period: oriC → terminus). Solution? Overlapping replication rounds: New forks fire at oriC before previous rounds complete, yielding 2–8 genome equivalents per cell during rapid growth.
Keywords: E coli genome replication time, bacterial overlapping replication, C period D period, multifork chromosome.
C, D, and τ Periods Defined
| Period | Duration | Description |
|---|---|---|
| C | 40 min | Chromosome replication (bidirectional forks, ~1000 kb each direction) |
| D | 20 min | Post-replication to division (segregation, septum formation) |
| τ | 20 min | Generation/doubling time (division interval) |
Formula: For τ < C, cells carry 2^(C/τ) origins. Here: 2^(40/20) = 4 origins/cell at division.
Visual Timeline: Multifork Magic
t=0: [oriC]---(forks start)---[terC]
t=20: Cell divides → Daughter1: [oriC]---half---[terC/2]
Daughter2: Same
t=40: Both complete replication + divide
Pro tip: Measure chromosome equivalents via flow cytometry—rapid growth shows >2N DNA content.
Exam Traps to Avoid
-
“20 min = replication”: Confuses τ with C [option A].
-
Ignoring multifork: Thinks division waits for replication.
-
Forgetting lab conditions: Poor media → τ ≈ C ≈ 60 min.
Practice Variations
-
τ=30 min, C=40 min: Still overlapping (1.33 rounds).
-
Minimal media: τ=60 min, C=40 min (no overlap needed).
Related: Helmstetter-Cooper model, DnaA/oriC regulation, SeqA sequestration.
